It is known that in the field of medicine there are substrates which are coated with high viscosity materials. For certain purposes it is sensible that these coatings do not generate a sealed surface but are applied as dots, which for instance allows sweat and other elimination products to escape from skin under bandages and not cause maceration. An adequate method of achieving this dotted coating is offered by rotational screen extrusion.
In this method a rotating screen has a nozzle located inside it, and through the nozzle the liquid that is to be applied is brought from outside to inside the screen space. It is then extruded out through holes in the screen in the direction of the substrate that is to be coated. Dependent on the substrate transport speed (rotational speed of the screen drum), the screen is lifted up by the substrate. Depending on the adhesion and internal cohesion of the liquid, the slugs which have already swelled so as to adhere to the supporting material draw out the limited stock of hot melt adhesive in the hole to a sharp contour, assisted by the sustained extrusion pressure, on to the supporting material.
On completion of this transport there forms, depending on the rheology of the liquid, over the pre-determined basis area a more or less heavily crumpled domed surface of the slug. The height to base ratio of the slug depends on the hole diameter to drum screen wall thickness ratio, and on the physical characteristics (flow behavior, surface tension and wetting angle on the supporting material) of the liquid.
Regarding substrate materials many types are prescribed and have been used in practice, including films, woven fabrics, knitted textiles, fleeces, gels and foams. In the medical sector there are particular requirements for the supporting materials. The materials must be compatible with the skin, generally permeable to air and/or water, easily formed and ductile. Based on these requirements, often the thinnest and weakest supporting material is preferred. For handling and use the supporting material must however be sufficiently strong and if necessary have only a limited tendency to stretch. Furthermore the supporting material should exhibit sufficient strength and limited tendency to stretch, even when wet through.
The arrangement of nozzle and screen are described in essentials in CH 648 497 A5, improvements to the method are described in EP 0 288 541 A1, EP 0 565 133 A1, EP 0 384 278 A1 and DE 42 31 743 A1.
For coating supporting material with subsequent medical, cosmetic or technical applications, it is preferable to use adhesives, and particularly preferable to use self-adhesives. It is preferable that these belong to the materials classes of solutions, dispersions, pre-polymers and thermoplastic polymers.
It is advantageous to use thermoplastic hot-melt adhesives based on natural and synthetic rubbers and on other synthetic polymers such as for example acrylates, methacrylates, polyurethanes, polyolefins, polyvinyl derivatives, polyesters or silicones, with corresponding additional materials such as adhesive resins, plasticisers, stabilisers and other additives as required.
Their softening point should be higher than 50° C., the application temperature is generally at least 90° C. and preferably between 100° C. and 180° C., or between 180° C. and 220° C. in the case of silicones.
The method therefore requires that the hot-melt adhesive is heated to a corresponding temperature to melt it so that it can run through the screen holes. It is usual for the hot-melt adhesive to be delivered from the feed system already melted, and to be kept in the nozzle at the corresponding temperature. In so doing it is generally attempted to maintain as high a temperature as possible, so that the viscosity of the adhesive remains low thus permitting a higher production speed. There are however tight limits to this method, since at excessive temperatures a rapid process of chemical decomposition takes place in the hot melt, which above all in medical coatings where contact with the skin will occur, is unacceptable.
So as to subject the hot melt adhesive to heat stress for as little time as possible, and thus minimize chemical decomposition, there exist various possibilities which essentially indicate that the screen should be heated so that the adhesive is kept warm in the critical zone of transit through the screen holes, and the risk of chilling avoided.
In the screen or around the screen there can be arranged for instance heater elements which act as radiant heat sources (EP 0 288 541 A1). Heating using hot air has also been described (CH 648 497 A5). These types of heating have however the disadvantage that not only the screen is subjected to radiant energy, but because of dispersion and the permeability of the screen for radiation and air flows, so also are the surroundings and the substrate to be coated.
A further method has been described in which the screen itself serves as heat source, by means of acting as a resistance in an electrical circuit (EP 0 384 278 A1). This requires however wide-ranging constructional features in the machine, so as to insulate the rotating screen electrically from the rest of the machine.
This method also exhibits weaknesses in continuous operation: The rotating screen, operating under rotating screen pressure, is mechanically not very stable, and during prolonged operation this leads to torsional strains with formation of associated bulges. In that circumstance parts of the screen touch the nozzle, which for process technical reasons cannot be insulated, and short circuits occur.
A further disadvantage of this arrangement is that the screen, in areas where it is not in contact with the substance, the substrate to be coated and the nozzle, is significantly more intensely heated than in areas where such contact does occur, and where the substance conveys the heat away. Temperature variations of 40 to 60° C. generally occur. This causes zonal mechanical weakening of the screen material by embrittlement due to overheating. The areas around the margins are most affected by this. The consequence is that in particular at high production speeds there occur damaging fractures of the screen.
The situation of screen heating up until now is characterized above all by the main attention being given to as even as possible heating of the screen over its entire area. This is solved almost ideally by the above mentioned resistance heating. For hot air heating, this objective is pursued using a screen with an enveloping hood (CH 648 497 A5), and for radiant heating by the use of multiple heating elements along the surface.
Disadvantages in heating the entire envelope are that one the one hand chemical decomposition occurs in the thin film of adhesive that remains on the screen and is carried round on its surface, because of the combination of large surface/volume ratio and therefore large contact area with ambient atmospheric oxygen; and on the other hand there is unnecessary loss by radiation into the ambient of part of the energy that is supplied.
Also current technology is that there should be energy introduced into the pulled material where the slug separates from the screen, so as to melt off any strings formed during separation from the screen and prevent formation of long strings (CH 648 497 A5; DE 39 05 342 A1). This is often necessary because the screen can cool off too rapidly after the main part of the slug of adhesive has passed through, and thus the viscosity of the remaining adhesive is increased to the point where string formation can occur. Heating the entire envelope according to current technology provides insufficient energy density at this point, or due to the geometrical configuration the energy cannot be applied sufficiently to the point of separation of adhesive from screen to compensate for the screen cooling off at this point. Therefore some string melt off device as described above is necessary.